Misplace medium Hebridean island (6)
I believe the answer is:
mislay
'misplace' is the definition.
('mislay' can be a synonym of 'misplace')
'medium hebridean island' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'hebridean island' becomes 'islay' (I've seen this before).
'm'+'islay'='MISLAY'
(Other definitions for mislay that I've seen before include "Put in wrong place, lose temporarily" , "Set in wrong location" , "Put something aside and forget where" , "Be unable to find" , "Lose track of" .)